The mechanism by which truncated kinesin dimers hydrolyze ATP and move unidirectionally along microtubules is well understood. It is far less clear how the full kinesin heterotetramer, which has two heavy chains and two light chains, is regulated and activated for cargo transport. In this work, we will test the hypothesis that kinesin is regulated when the tails directly bind the heads to prevent ADP release or microtubule binding. Kinesin may be further regulated by a charge clash between its light chains and microtubules, and kinesin may be re-activated when phosphorylated light chains compete the tails away from the heads. We will test these hypotheses in four Specific Aims. The first two Aims address regulation using the full-length kinesin heavy chain, and the second two Aims explore the role of the light chains in regulation and activation. In Aim #1, we will determine whether the tail binds directly in the microtuble-binding site or to the nucleotide-sensing elements in the head, or whether it allosterically affects the nucleotide- or microtubule-binding regions of the head. The experiments of Aim #2, guided by the results of Aim #1, will determine what region of the head binds the tail and will identify specific head-tail interactions. Experiments performed both in vivo and in vitro indicate that the light chains may have a significant role in regulating kinesin, which will be assessed in Aim #3. Lastly, we will determine whether phosphorylation of kinesin light chains can directly activate kinesin in Aim #4. Together, these experiments will extend our understanding of the interactions and conformational changes that govern kinesin activity. Furthermore, the regulatory interactions that are found in this work may reveal inhibitory mechanisms that are similar in several kinesins. This may lead to quicker discovery of drugs that specifically target kinesins.